Micro-electronic circuit components for most portable electronic items, such as integrated circuits and electronic displays, are commonly installed within flat, hard surfaces of electronic devices, such as a rigid IC substrate or “chip”, computer screens, television sets, smart phones, tablet computers, etc., and in many cases are installed on accessories for the electronic devices, such as removable monitors. Many electronic devices having an electronic display or other micro-electronic circuit are portable, and have thus become very useful in implementing mobile applications. This fact is particularly true with smart phones which have become ubiquitous. However, typical mobile devices such as smart phones have electronic displays that are rigid (and in some cases, flat) in nature. Thus, while these displays are useful in implementing many different applications, the device on which the display is present must still typically be held in a hand, or must be stored in a pocket, a purse, a briefcase or other container, which makes the electronic device less accessible in many situations, such as when a person is carrying other items, undertaking an athletic activity such as running, walking, etc. Moreover, in many cases these traditional electronic devices require two free hands to hold and operate, making these devices cumbersome or difficult to use or to view in situations in which, for example, a person has only one or no free hands or is otherwise occupied.
Flexible micro-electronics circuits, such as displays, are generally known and are starting to come into more common usage. However, flexible displays have not been widely incorporated into easily portable items such as items of clothing, wristbands, jewelry, etc. or on items that are easily attached to other items, much less in a manner that makes the display more useable and visible to the user in many different scenarios. Flexible displays are often thin sheet-like, band-like, or web-like structures, having a comparatively small thickness or height relative to a length and a width, and formed of one or more substrates, layers, and/or plies.
Dynamically flexible displays are also known. A dynamically flexible display is a type of flexible display that is adapted to be flexed, bent, shaped, and re-flexed and re-shaped multiple times. The terms “dynamic bending,” “dynamic flexibility,” “dynamically flexible,” and similar terms, for example, generally refer to the ability to bend the attachable article, and more particularly the flexible electronic component, at a number of different points, if not every point, along a length of the flexible component, the ability to bend different portions of the attachable article differently (e.g., different portions can be bent at/to different angles or curvatures), the ability to bend the attachable article in a number of different directions (e.g., in a concave direction and a convex direction), and/or the ability to bend the attachable article in some other dynamic manner. Further, dynamically flexible may refer to the ability to repeatedly bend (i.e., multiple times) in different manners and directions, for example, so to as be selectively formable and re-formable to various different shapes by a user, without damaging the intended operable usability of the article, such as would be usual and customary when used as a wrist band, belt, item of clothing, and the like.
As illustrated in FIG. 1. a typical flexible display has three main components: namely, an electro-optic display panel 20, a rear substrate 21 disposed on the rear side of the electro-optic display panel 20, for example to support the electro-optic display panel 20, and a front substrate 24, which may include an optional touch sensor 25 and adhesive layer 26, disposed on the front side of the electro-optic display and through which the electro-optic display panel is visible. The flexible display may optionally have a flexible support substrate including foam layers 22 and adhesive layers 23 disposed on a rear side of the rear substrate. As used herein, the “front” side of the display panel 20 is the side that produces changeable optical images intended to be seen by a viewer, and the “rear” or “back” side of the display panel is the opposite side, which typically is not designed to display any particular images to be seen by the user.
FIG. 2 illustrates a detailed cross-section of an exemplary stack of layers of a typical flexible display (hereinafter, also called a “display stack”). The electro-optic display panel 20 includes an electo-optic medium 20a and an electrode matrix 20b secured to a rear side of the electro-optic medium 20a. Typical electro-optic mediums include liquid crystal displays (LCDs), cholesteric texture liquid crystal (CTLC) displays, Polymer-dispersed liquid crystals (PDLC) display, E-Ink® displays, electrophoretic displays, and electrowetting displays. The electro-optic medium 20a creates and displays changeable visual information, which is visible on the front side of the electro-optic display panel 20, in response to electronic command signals received through the electrode matrix 20b. The terms “front” and “rear” are used throughout with reference to the front side and the rear side of the electro-optic display panel 20. The front substrate 24 and all the layers on top, i.e., everything on top of the electro-optic medium 20a (as depicted in FIG. 2), are simply called the “top substrate” 27. In this example, the top substrate 27 includes the front substrate 24 disposed against the front side of the electro-optic medium, a moisture barrier 28 disposed against the front side of the front substrate 24, an adhesive layer 26 disposed against the front side of the moisture barrier 28, and a touch sensor 25 disposed against the front side of the adhesive layer 26. Preferably, another optically clear adhesive layer is disposed between the electro-optic medium 20a and the moisture barrier 28 to adhere moisture barrier 28 to the electro optic medium 20a. The rear substrate 21 here is also called the “bottom substrate” 21 and is typically made of a single layer that holds the matrix electrodes and other electronic elements. This layer may be flexible enough to allow a desired flexibility of the overall flexible display while being resilient enough to provide support and/or protection for the electro-optic display panel 20. The bottom substrate 21 may not be the bottom-most layer, however. Rather, a flexible support substrate may be attached to the rear side of the bottom substrate 21, including a foam layer 22 and an adhesive layer 23. The adhesive layer 23 is disposed against the rear side of the bottom substrate 21 and the foam layer 22 is disposed against the rear side of the adhesive layer 23. The foam layer 22 is a relatively thick layer of foam having visco-elastic properties and a very low stiffness when flexed slowly that increases when the foam is indented with high speed. This feature results in the foam layer 22 not affecting the flexibility of the display to any significant respect, but at the same time protects the display from impact by effectively spreading an impact over a larger area.
FIGS. 3A-3C illustrate a diagrammatic and schematic overview of a typical active-matrix thin film transistor (TFT) liquid crystal display (LCD) 30. In this example, the LCD includes an ITO (indium tin oxide) circuit on glass and a color filter array forming a top layer 31, an array of ITO electrodes and switches on glass forming a bottom layer 32, and a liquid crystal 33 disposed between the top layer 31 and the bottom layer 32. FIG. 3B illustrates an enlarged detail view of the bottom layer 32, which forms an active matrix circuit. FIG. 3C illustrates an electrical schematic of a basic circuit of the pixel 36, including a switch 37 and a liquid crystal element 38, formed by the active matrix circuit for one pixel of the LCD 30. The active matrix circuit of the bottom layer 32 includes row (gate) electrodes 34 and column (source and/or drain) electrodes 35, which together form a grid with one row electrode 34 and one column electrode 35 connection per pixel 36 of the LCD 30. The row electrodes 34 are typically electrically isolated from the column electrodes 35 by a dielectric layer (not pictured) and are attached to opposite sides of a circuit element such as a switch, which may be a transistor, an inverter, or other circuit element, used to control or initiate light to be emitted from the display. For example, the row electrodes 34 may be disposed at a first level of a layer stack, the column electrodes 35 may be disposed at a second level of the layer stack, and the dielectric layer may be disposed between the first level and the second level to electrically separate the row electrodes 34 from the column electrodes 35, although other functionally equivalent arrangements are also common. The switch 37 of a particular pixel 36 is connected to and disposed electrically between the row electrode 34 and the column electrode 35 associated with that pixel. In this example, the switch 37 is a transistor with a gate connected to the row electrode 34 and a source and drain connected to the column electrode 35. However, other types of switches and specific circuit arrangements are possible. When appropriate drive signals, such as selected electrical currents or voltages are provided to each of the row electrode 34 and the column electrode 35 of a particular pixel, the switch 37 is energized or thrown, thereby activating the display element (e.g., the liquid crystal element 38) for that pixel 36. As can be readily understood, when one of the electrodes 34 or 35 is interrupted, for example by a crack, the electrical signal cannot be passed to other pixels connected to that electrode after the interruption, resulting in a partial line defect. Such a defect can impair or prevent functioning of the LCD 30.
FIGS. 4A-4E illustrate a layer by layer overview of a typical layer stack (a “TFT stack”) 40 for a pixel 35 in a typical active-matrix LCD, such as the LCD 30. In this case the TFT stack 40 is a bottom-gate, bottom-contact transistor configuration (a “bottom gate bottom contact stack”) without a storage capacitor per pixel. However, other layer stacks are also possible, such as a top contact stack, a top gate stack, and combinations thereof. In the layer stack 40, the gate electrode 34 is the first layer of the TFT and is disposed on a bottom substrate 41, such as glass, as seen in FIGS. 4A and 4B. Next, as shown in FIG. 4C, a dielectric layer 42 is disposed on the gate electrodes 34 and the bottom substrate 41. In FIG. 4D, the source-drain electrodes 35 and a pixel pad 43 are disposed on top of the dielectric layer 42. FIG. 4D shows a semiconductor layer 44 disposed on top of the source-drain electrodes 35, thereby forming the complete layer stack 40. In some cases additional layers are used, such as etch stop layers to stop etching of the top metal contacts on top of the semiconductor.
The metal lines defining the gate and/or column electrodes 34 and 35 typically will break in the same region (range) of strain as other brittle layers in an LCD stack, such as the layer stack 40. This breaking point is typically at around 1% strain, either tensile or compressive. However, this range can be somewhat larger when the electrodes 34 and 35 are patterned layers and/or when ductile metals are used, such as gold. Nevertheless, it is undesirable to strain the metal of the row and/or column electrodes 34 and 35 beyond its yield strength and into its plastic deformation region, because the strain is not reversible in that region.
Moreover, there are several scenarios where the strain limits of the layers can be reached such that undesirable cracking and/or formation of gaps in the electrodes 34 or 35 may occur. A first scenario occurs during bending of the entire display 30, in particular including bending of the bottom substrate 41, such as during dynamic bending, where this bending imposes a certain limit on the overall bending radius of the LCD 30. A second scenario, as shown schematically in FIG. 5, occurs during a relatively sudden impact on the display, during which the object impacting the display creates a spot where the individual layers of the layer stack 40 are stretched in only a small region of the overall LDC 30. FIG. 5 illustrates a schematic cross section of the LCD 30 when impacted by a spherical ball, for example. When the electro-optic medium of the display, such as the liquid crystal 33, is either a fluid or a polymer with fluidic pockets, capsules or a polymer network in a fluid (LCD, CTLC, PDLC, E Ink, electrophoretics, electrowetting), this medium will have a Young's modulus that is orders of magnitude lower than that of either plastic or glass, and therefore in general also has a much lower flexural rigidity than either plastic or glass.
For a flexible electronic display or other flexible electronics article including a layer stack 40 as outlined above, the preferred materials from which the electrodes 34 and 35 and other portions of the circuit are made (e.g. metals like Au, Ag, Cu, ITO, Mo, Al) are often brittle, i.e., have low ductility, and therefore are subject to cracking and breaking under strains developed, for example, when the electronics circuit undergoes dynamic bending or a sudden impact as described above. Such cracks can form gaps along the electrodes 34 and 35, for example on the order of 1 nanometer to 500 nanometers and even up to 1 μm, which can degrade or even prevent conduction of electrical current through the electrodes 34 and 35, and thereby degrading or preventing proper functioning of the circuit. Although more ductile conductors are known, which would be able to withstand more strain and/or sudden impact without developing such gaps, these conductors tend to have lower electrical conductivity and, therefore, are not preferred for use as the conductors 34 and 35 for reasons related to speed and efficiency (power usage) of the circuit. Thus, a limitation on the production and/or durability of a typical micro-electronics circuit is the amount of strain that can be endured by the conductors 34 and 35 without degrading or preventing functioning of the circuit according to its intended capabilities.